The overall goal of this project is to improve the depiction of the vascular lumen using Contrast-Enhanced Magnetic Resonance Angiography (MRA). The injection of contrast material into the circulation system produces changes in the relaxation times of blood that depend on the intrinsic properties of the contrast agent, and in particular on the concentration of the agent. Because of flow dynamics, the concentration at any particular point in the vasculature varies substantially over time. Similarly, at any given point in time, the concentration shows profound spatial variations. 3D MRA data acquisition generally cover ten cardiac cycles, or more. The magnetization distribution in vessels through the sampled volume changes through the acquisition cycle, and the reconstructed image therefore represents a time averaging of the distribution. While the resultant images that might appear to show vessels with good contrast to noise, the true fidelity of this approach requires a more careful examination. We propose to develop a comprehensive numerical package to simulate the passage of contrast material through representations of vascular territories of high clinical interest. This will include, but will not be limited to, the carotid bifurcation, and the intracranial aneurysms. The simulations will provide an accurate representation of the spatial/temporal variation of magnetization in those regions, and will provide an accurate representation of the spatial/temporal variation of magnetization in those regions, and will permit it the reconstruction of MRA images for pulse sequences currently employed, and for novel sequences as they arise. Flow conditions ranging from laminar through transitional to fully developed turbulent flow will be simulated. The simulations will provide a framework in which to evaluate the relative merits of different pulse sequence designs. The predictions of the simulation models will be verified in a series of carefully controlled flow model experiments. These will use geometries that correspond exactly to those in the simulation, and that are realistic representations of diseased vessels. These experiments will validate both the hemodynamic aspects (using Laser Doppler Velocimetry) and the MRA aspects (using MR imaging methods) of our simulations. The sequences and methods considered to be most promising for in vivo studies, as demonstrated by simulation and experiment, will be evaluated in patients already identified as having vascular disease. It is anticipated that this project will provide improved tools for the understanding of Contrast-Enhanced MRA and will lead to more accurate and reliable methods for assessing vascular disease in vivo.